1. Field of the Invention
The present invention relates to a process for forming contact through-holes in a insulating layer of a semiconductor device such as an integrated circuit (IC).
2. Description of the Prior Art
In the production of a semiconductor integrated circuit, a portion of an insulating layer for protection (i.e. a passivation film) is selectively removed to make a through-hole (i.e. a via hole) and then an upper conductor layer (i.e. a conductive pattern) is formed on the insulating layer so as to come into contact with a lower conductor layer (i.e. another conductive pattern) through the through-hole. In this case, it is possible to generate cracks in the upper conductor layer (e.g. aluminum) at the edge of the insulating layer defining the through-hole. As a result, a disconnection i.e., a discontinuity between the upper and lower conductor layers may occur. Accordingly, in order to prevent cracks from occurring, it is important to eliminate the edge of the insulating layer at the juncture of the sidewall defining the through-hole and the upper surface of the insulating layer, and instead to shape the side surface of the through-hole into a slope.
Various etching methods for forming a through-hole having a slope have been proposed. For example, a portion of an insulating layer on a conductive layer can be selectively removed through an opening in a patterned photoresist mask by using an etching solution to form the through-hole. Such a wet-etching method is explained in detail, referring to FIGS. 1 and 2. An insulating layer (e.g. a phosphosilicate glass film) 1 is formed on a conductor layer (e.g. an aluminum pattern) 2 lying on another insulating layer (e.g. a silicon dioxide layer) 3 and a semiconductor substrate (e.g. a single crystalline silicon substrate) 4. A photoresist layer 5 is applied on the insulating layer 1 and then is exposed and developed to form a pattern mask having an opening 6, as illustrated in FIG. 1. A portion of the insulating layer 1 (e.g. of phosphosilicate glass) is selectively etched by using a suitable etching solution (e.g. a mixed solution of hydrofluoric acid (HF) and nitric acid (HNO.sub.3)) to form a through-hole 7, as illustrated in FIG. 2. The through-hole 7 has a larger bottom size and a considerably larger top size thereof than the size of the opening 6 of the photoresist layer 5, since the etching solution can remove the insulating layer 1 from under the edge of the photoresist layer 5 (namely, a so-called undercut or side etching occurs) and over-etching is practically carried out to ensure formation of a number of complete through holes in a number of semiconductor device chips of a semiconductor substrate (wafer). In this case, it is possible to prevent the cracks of an upper conductor layer to be formed from occurring; however, the formed through-hole is made larger than the desired size, so that the density of the conductor pattern and the density of the integrated circuit are low.
By decreasing the undercut, a contact through-hole can be formed by combining wet-etching and dry-etching. An exposed portion of the insulating layer 1 (FIG. 1) is wet-etched by the etching solution through the opening 6 of the photoresist layer 5. For example, as shown in FIG. 3A, the insulating layer 1 having a thickness of 1.0 micron is wet-etched to form a preceding hole 8 having a depth of 0.7 microns; as a result, a portion of the insulating layer having a thickness of 0.3 microns remains. Then, in a conventional dry-etching (e.g. reactive sputter etching, plasma etching) apparatus, the insulating layer 1 is further etched to complete a through-hole 9, as illustrated in FIG. 3B. The formed through-hole 9 has a smaller bottom size and a slope of a larger angle as compared with the through-hole 7 (FIG. 2) formed by wet-etching only. In this case, since there is a space between the bottom plane of the photoresist layer 5 and the sloped sidewall (inclined plane) of the preceding hole 8, dry-etching causes partial removal of the insulating layer 1 from under the photoresist layer 5, resulting in a small undercut. Thus, the through-hole 9 is slightly larger than the desired size.
In another case, the insulating layer 1 having a thickness of 1.0 micron is wet-etched to form a preceding hole 10 having a depth of 0.3 microns, as illustrated in FIG. 4A. Then, in the same dry-etching apparatus as in the case of FIGS. 3A and 3B, the insulating layer 1 is further etched to complete a through-hole 11, as illustrated in FIG. 4B. The formed through-hole 11 has an almost vertical side surface and only a short portion having a slope, formed initially by wet-etching. Since the through-hole 11 does not have a slope extending over the whole depth of the hole, cracks may occur in a conductor layer (not shown, but formed by a conventional process) formed on the insulative layer 1 at the edge of the through-hole 11, where that conductor layer extends through the through-hole 11 to contact an exposed portion of the conductor layer 2.